Unannealed nickel screen grid mesh for pickup tubes



Dec. 27, 1966 J. G. ZIEDONIS 3,29

UNANNEALED NICKEL SCREEN GRID MESH FOR PICKUP TUBES Filed Feb. :5, 1964I NVENTOR. JIM/5 & Z /00/V/5 OMKJW United States Patent 3,295,0il6UNANNEALED NICKEL SCREEN GRED MESH FGR PICKUP TUBES Janis G. Ziedonis,Lancaster, Pa., assignor to Radio Corporation of America, a corporationof Delaware Filed Feb. 3, 1964, Ser. No. 342,089 6 Claims. (Cl. 313-269)This invention relates to pickup tubes and in particu velope and facingthe electron gun positioned in the other end portion of the envelope.The target electrode includes a transparent conductive coating or signalelectrode on the gun side of the transparent support member, and aphotoconductor comprising a planar deposit of photoconductive materialon the transparent conductive coating. Photoconductive materials arematerials which undergo a change in their electrical conductivity inresponse to incident radiations. These materials have a relatively highelectrical resistance when in the dark and a relatively high electricalconductivity when exposed to light or other radiations of a selectedfrequency.

Closely spaced from the exposed planar surface of the photoconductivematerial in the direction of the electron gun, is a fine mesh screenelectrode. The fine mesh screen electrode is substantially planar and ispositioned during manufacture of the tube so as to lie substantiallyparallel to the exposed planar surface of the photoconductive material.The preservation of such parallel disposition of the mesh screen isdesirable during operation of the tube so that the screen may performits function without adverse effects on the output of the tube. Suchfunction involves creating a uniform field between the target and meshfor causing the electron beam to be perpendicular to the target onstriking it.

Adverse effects are likely to arise if the mesh screen is caused tovibrate with appreciable amplitude during operation of the tube. Suchamplitude vibrations may be of the resonant type excited by relativelysmall mechanical shocks applied to the tube during operation.

The relative movement between the target and mesh involved in suchvibrations results in variation of the effective capacitance between themesh and target. This variation of the effective capacitance isreflected in a corresponding variation in the picture signal outputlevel.

A contributing fact-or to the problem of mesh vibration has been thebelief heretofore that the composition of the mesh should be restrictedto a non-magnetic material such as copper and that the mesh should beheated to a relatively high temperature during processing. A priorpractice of making an assembly including a mesh mounted on a supportring, has involved the following steps. A copper mesh was first looselyextended across the support ring and then welded thereto withoutattempting to tighten the mesh mechanically. For accomplishing atightening of the mesh on the support ring and also for cleaning themesh, the assembly was then heated in a furnace to about 780 C. However,in spite of the attempts to tighten the mesh by heating, the mesh havingthe foregoing composition and processed as indicated, was characterizedby appreciable vibration during operation of a tube in which it wasincorporated.

This is believed due to the fact that copper begins to stress relieve ata relatively low temperature, i.e., about 200 C. Therefore, heating thecopper to a temperature of about 780 C. as required in prior methods,fully stress-relieved or annealed the copper, thereby enlarging itscrystals and causing a reduction in its internal damping property.

Accordingly, it is an object of the invention to provide an improvedpickup tube.

Another object is to provide a pickup tube having an improved meshscreen in which vibration is minimized.

A further object is to provide an improved method of making such a meshscreen assembly.

The foregoing objects are realized in a pickup tube having a mesh screenassembly using a screen which is unannealed and stretched across thesupporting ring prior to being welded thereto. The subsequent welding ofthe mesh to the ring completes the mesh-ring assembly. Heating of theassembly is deliberately avoided.

Further, instead of copper as heretofore employed, the mesh screen ismade of a material such as nickel. Although this material is magnetic,applicant has found that the magnetic character of the metal is notobjectionable.

This preference for a material such as nickel is a consequence of thefact that the annealing temperature of this metal is much higher thanthe normal tube processing' and application temperatures. Stretching thescreen mesh across the support ring prior to welding the screen mesh tothe support ring and without annealing the screen assembly has asignificant bearing on screen vibration. It is believed that screensmade according to this invention provide internal damping of the meshassembly, thus considerably reducing the decay time of vibration.

A vibration of the mesh screen at a relatively small amplitude or avibration having a relatively fast amplitude decay, is free from anyappreciable harmful effects in the output of the tube. Applicant hasfound that his improved mesh screen is characterized by such a fastdecay of vibration amplitude that it is not detectible after a period ofabout 0.1 second on a monitor of a camera. Applicants mesh screen istherefore of appreciably greater advantage than prior copper meshscreens.

In the drawing which illustrates an embodiment of the invention,

FIG. 1 is a side view, partly in section, of a vidicon type of pickuptube in which the invention is used;

FIG. 2 shows a sectional view of a mesh screen assembly employed in thetube of FIG. 1;

FIG. 3 shows a jig in cross-section, employed in carrying out one stepof the method of the invention; and

FIG. 4 shows in cross-section the jig of FIG. 3 as well as an additionaljig employed in another step of the method of the invention.

Referring to the drawing, in FIG. 1 there is shown a photoconductivetype pickup tube 10 which is conventional except for the improvementsembodied therein in accordance with this invention as will appearhereinafter. The tube 10 comprises an elongated evacuated envelope 12having in one end thereof an electron gun 14 for producing an electronbeam. The electron gun 14 includes a cathode electrode 16, a cup-shapedcontrol electrode 18, a first accelerating electrode 20, a secondaccelerating electrode 21 and a final accelerating electrode 22. The endof the accelerating electrode 22 remote from the gun 14 is terminated byan apertured fine mesh screen electrode 24. The electrode assemblyforming the gun 14 is supported by lead-in pins 25 that extend through astem 26 that forms an end of the envelope 12 and the spring spacerelements 23. The stem 26 also includes an exhaust tabulation 27.

The other end of the envelope 12 includes a lighttransparent faceplate28 made of a material such as glass and having a target electrode 29 onthe inner face thereof.

The target electrode includes a layer 30 of photoconductive materialsuch as porous antimony trisulfide deposited over a conductingtransparent layer or signal electrode 32. The conducting layer 32 mayconsist of tin oxide. The screen electrode 24 is relatively closelyspaced with respect to the target electrode 29, i.e., .050 inch.

As shown in more detail in FIG. 2, the mesh screen electrode 24 ismounted in stretched condition across a circular mounting ring 40. Themesh screen electrode is made of a metal in which the internal stressesthereof are preserved during processing and operation of tube 10. Suchmetal may be nickel in relatively pure form, such as electrolyticnickel. The ring 40 is made of a metal that is characterized by theseveral properties of being non-magnetic and substantially free fromoxidation, and contributing to the formation of a good braze or weld.One material that applicant has found suitable for the ring 40 is analloy known under the trade name of Nichrome. This alloy consists ofnickel, iron and chromium. The mesh screen electrode 24 is fixed to theoutwardly extending flange 42 of the mounting ring 40 by suitable meanssuch as a body of brazing material 44 made of Nichrome.

The assembly comprising the screen electrode 24 fixed to the supportring 40, is mounted on the end of the final accelerating electrode 22adjacent to the target electrode 29 (FIG. 1). The mounting of the screenassembly on electrode 22 is accomplished by a forced fit only. Themounting is facilitated by means of embossments 43 extending inwardlyfrom the electrode 22 (FIG. 2). Three or more embossments may beemployed. The limited area contacts provided by the embossments absorbthe entire of a moderate force applied to the screen and ring assemblywhen a portion of the assembly is telescoped into one end of theelectrode 22, as shown in FIG. 2. This results in a relatively highpressure per unit area between the embossments 43 and the ring support40. Such high pressure is adequate to prevent displacement of the screenand ring assembly from the electrode 22 during operation of the tube 10.This type of mounting is advantageous from several standpoints. Itcontributes to economy in tube fabrication and avoids harmful effectsfrom heat required in welding or brazing.

The assembly comprising the screen 24 and the support ring 40 is made ina convenient manner by utilizing jigs shown in FIGS. 3 and 4. The jigshown in FIG. 3 comprises two metal rings 46, 48 adapted to be clampedtogether by spring clamps 50 fixed to the outer periphery of ring 48 andurged into recesses 52 in the outer periphery of ring 46. A screenworkpiece 54 positioned between jig rings 46, 48 is held firmly when theclamps are seated in the recesses 52. To assure a coplanar dispositionof the central portion of the screen workpiece 54 with the portionthereof engaged by the rings 46, 48, the jig includes a support 56having a relatively thin annular portion 58 on which the ring 48 isadapted to rest. The central portion 60 of support 56 is sufficientthick so that its upper surface, as viewed in FIG. 3, is in the plane ofthe upper surface of the ring 48 when this ring rests on the annularportion 58. In this way the screen workpiece 54, when laid over the ring48 and the thick portion 60 of the support, is disposed in a singleplane. It is found, however, that after the support 56 is removedsubsequent to clamping the screen workpiece 54 by the rings 46, 48, aslight dropping displacement of the central portion of the screenworkpiece occurs. Such displacement is objectionable in the completedscreen electrode inthat when this electrode is incorporated in the tubethe displacement will adversely affect a desired uniformity in spacingbetween the screen electrode and the target electrode 29.

To avoid such displacement of the screen workpiece 54 prior to affixingthereof to the flange 42 of the support ring 40, the jig shown in FIG. 4is employed. This jig comprises a base 62 from which a cylinder 64extends upwardly. The cylinder 64 has an inner diameter for looselyreceiving therein the cylindrical portion 66 of the screen support ring40, and supportingly engages the under-side of flange 42 of the supportring. When the screen workpiece 54 while clamped between the rings 46,48, is laid over the flange 42 of the support ring, and the rings 46, 48are relieved of support, the combined Weight of the rings which may begrams, is suflicient to stretch the screen workpiece 54 a desirableamount, so that the entire portion of the screen workpiece within thecircle defined by the flange 42 is in one plane when supported solely bythis flange It has been found that a stretching which produces afrequency response, of, for example 5000 -c.p.s., results in asatisfactory screen.

While the screen workpiece 54 is so supported by the flange 42, arelatively thin brazing washer is placed over the portion of theworkpiece engaged by the flange 42. The brazing washer 68 may be made ofNichrome. Successive portions of the brazing washer 68 are heated to themelting temperature of the washer i.e. 1400 C., for brazing the screenworkpiece 54 to the flange 42. The heating may be accomplishedelectrically by means of an elect-rode 70 connected to a suitableelectric power supply 72 as shown in FIG. 4. To avoid weld splash, thepower density used is about 10 Watt seconds.

The localized character of the heating and the appreciably large thermalreservoir provided by the support ring 40 results in the avoidance ofany harmful heating of the screen workpiece 54 during the brazingoperation,

After the brazing operation has been completed, the screen workpiece 54is released from the clamping rings 46, 48 and the portion of the screenextending radially outward from the flange 42 is suitably removed. Aftersuch removal the assembly comprising the screen grid 24 and support ring40 is completed.

The advantage of using nickel, or similar metal, as the composition ofthe mesh screen will appear from the following.

Nickel has an annealing or stress relieving temperature of about 620 C.This temperature is appreciably above that reached by a pickup tubeeither during processing or operation. The highest processingtemperature of the tube is about 350 C. The operating temperature isappreciably lower. Thus a mesh screen made of nickel remains free fromstress relief and is characterized by a higher number of dislocations(various lattice imperfections) and relatively small crystal structure.

The different lattice imperfections are very important inre-establishing equilibrium or steady-state conditions in a solid suchas a screen mesh. Such imperfections are either point, or line defects.Vacant lattice sites and interstitial atoms are the principal pointdefects involved in migration phenomena through the material. Theprincipal line defects are the dislocations. These dislocations ariseduring crystal growth in manufacturing the mesh. These dislocations areimportant because of their significance in plastic behavior of crystalsand crystal boundaries.

By exposing the mesh to extremely high temperatures the number ofdislocations in the mesh material is reduced and the crystal structuretherein is considerably enlarged. Such reduction in the number ofcrystal dislocations is undesirable for the purposes of the presentinvention. This is because a high number of dislocation is necessary forhigh mechanical energy losses in the material.

The greater shock absorbing characteristic of unannealed material isbelieved to be due to the fact that external impact energy doeirreversible work on the dislocations and hence dissipate energy. Alsothe smaller crystal size contributes to higher internal damping. Thismay be explained in terms of the increased crystal boundary area definedby the smaller crystals. This increased crystal boundary area willincrease the internal energy loss through crystal boundary slip. In anannealed maten'al having relatively few dislocations, movement of thedislocations in the material are few and the impact energy cannot bedissipated fully in such movements. The only way that the impact energyapplied to a body of annealed material can be relieved is by movementsof the entire body in the form of relatively high amplitude vibrations.

One technique practiced by applicant in making the mesh screen workpiece54 has involved the following steps. A coating of Wax one-sixteenth ofan inch thick is applied to a smooth glass substrate. By means of aruling engine horizontal and vertical grooves are provided in the waxcoating. The grooves are of sutficient depth to expose portions of theglass substrate. In one example 1000 horizontal grooves and 1000vertical grooves were provided per square inch of area ruled, A smallernumber of grooves may be provided as desired. Thereafter palladium issputtered over the grooved surface, to substantially fill the grooveswith palladium. This also resulted in the formation of an unwanted thinpalladium coating on the remaining Wax portions of the grooved surface.Palladium was selected as the best material upon which to deposit nickelto form a nickel screen in accord ance with the invention, because ofthe ease with which palladium can be removed after it has served itspurpose. Thus after the palladium has been sputtered upon the groovedsurface the relatively thin coating thereof formed on the wax is easilyremoved after the coated substrate is immersed in deionized water. Oneway in which removal of the thin coating may be accomplished is bymerely moving the hand of an operator lightly across the coated surface.The relatively thin coating of palladium on the waxed portions of thegrooved surface is thus easily removed while the palladium in thegrooves is shielded by the wax and remains in the form of a palladiummatrix. The glass substrate having the palladium matrix thereon is nextsubjected to an electroplating operation for depositing a layer ofnickel over the palladium. The bath used in the electroplating operationmay include nickel sulfamate in a suitable solvent, into which thesubstrate having the palladium matrix thereon is immersed. Since allportions of the palladium matrix are electrically interconnected, as aconsequence of the integral structure of the matrix, one portion thereofadjacent to the edge of the substrate is electrically connected, as byan alligator clip, to the negative side of a current supply. Thepositive side of the current supply is connected to an anode, preferablymade of a nickel-carbon alloy, immersed in the plating bath. A 60 amperecurrent supply may be employed.

The substrate with the palladium matrix thereon is permitted to remainin the bath until a nickel coating having a thickness of about 0.2 milis deposited upon the palladium matrix. Thereafter the substrate havingthereon the palladium matrix coated with nickel, is removed from theplating bath and placed in a bath of deionized water. The matrix orscreen of nickel-coated palladium is then pulled manually from thesubstrate. The palladium is then removed from the nickel screen as byrubbing. The resultant nickel screen constitutes the screen workpiece 54shown in FIG. 3. The screen so processed has a transparency of 68%.

It will be noted in FIG. 4 that the screen workpiece 54 includes an edgeportion extending beyond the circle of the sup-port ring 40. This edgeportion forms no part of the active portion constituting the screen 24shown in FIG. 2. Therefore the edge portion of the screen 54 adverselyaffected by the engagement of the palladium matrix by the alligatorcontact during the plating operation, is removed after the screen 24 isfixed to the ring 40.

Constructing a screen in accordance with this invention results inscreen elect-rode for a camera tube in which the period of vibration ofthe screen (the decay time) is appreciably reduced thus providingimproved operation of the tube in which the screen is used. An improvedmeth- 0d of making such a screen electrode is also provided.

What is claimed is:

1. A pickup tube having:

(a) a planar screen grid,

(b) said screen grid including a tautly supported perforated planarmetal mesh structure,

(c) the metal of said structure being unannealed and having an annealingtemperature higher than the processing and operating temperatures ofsaid tube,

(d) said structure being uncoated,

(e) whereby said screen grid is characterized by a relatively highinternal damping of vibrations.

2. A pickup tube having:

(a) a planar screen grid,

(b) said screen grid comprising a metal ring and a metal mesh screenstructure tautly supported across said ring,

(c) said planar metal screen consisting of bare unannealed nickel havingan annealing temperature higher than the processing and operatingtemperatures of said tube,

(d) whereby said structure is adapted to dampen vibrations thereof.

3. A pickup tube having:

(a) a planar target,

(b) a planar mesh screen grid adjacent and parallel to said target,

(1) said screen grid including a screen portion of uncoated unannealedmetal,

(2) said metal having an annealing temperature higher than theprocessing and operating temperatures of said tube,

(3) whereby vibrations of said screen portion are damped for preservinga substantially constant predetermined spacing between said target andgrid.

4. A pickup tube having:

(a) a target,

(b) a screen grid closely spaced from said target,

(1) said screen grid including a metal support ring,

(2) a perforated metal mesh structure tautly supported across said ring,said metal mesh structure being free of any coating,

(3) said metal structure consisting of unannealed nickel having anannealing temperature higher than the processing and operatingtemperatures of said tube,

(4) whereby vibrations of said metal structure are damped for preservingconstant the relatively close spacing between said grid and target.

5. A pickup tube having:

(a) an elongated envelope,

(b) a planar target in one end portion of said envelope,

(c) an electron gun in the other end portion of said envelope, and

(d) a planar screen electrode intermediate said target and electron gun,

(1) said screen electrode being parallel and adja cent to said targetand including an uncoated mesh screen portion consisting of anunannealed metal,

(2) said metal having an annealing temperature substantially higher thanthe highest temperature to which said tube is subjected duringprocessing and operation.

6. A pickup tube having:

(a) an elongated envelope,

(b) a planar target in one end portion of said envelope,

(c) an electron gun in the other end portion of said envelope, and

(d) a planar mesh screen electrode intermediate said target and electrongun, (1) said screen electrode and said target electrode being in tworelatively closely spaced and paralr lel planes,

(2) said screen electrode being made of unannealed and uncoated metalhaving an annealing temperature substantially higher than the processingand operating temperature of said tube,

(3) whereby high amplitude vibrations of said screen electrode aredamped and said close spacing between said target and screen electrodeis preserved constant.

References Cited by the Examiner UNITED STATES PATENTS 2,538,836 1/1951Jensen 31389 8 2,547,638 4/1951 Gardner. 2,738,436 3/1956 Zaphiropoulos.2,878,540 3/ 1959 Willner 2925.14 2,901,649 8/ 1959 Knight 313682,926,419 3/ 1960 Harris 313-348 X 2,930,718 3/1960 Coflin. 2,951,9629/1960 Miller et a1. 313-89 2,979,633 4/1961 Harris 31389 3,042,992 7/1962 Weissfioch 2925 .14

0 JOHN W. HUCKERT, Primary Examiner.

DAVID J. GALVIN, A. J. JAMES, J. D. KA-LLAM,

' Assistant Examiners.

1. A PICKUP TUBE HAVING: (A) A PLANAR SCREEN GRID, (B) SAID SCREEN GRIDINCLUDING A TAUTLY SUPPORTED PERFORATED PLANAR METAL MESH STRUCTURE, (C)THE METAL OF SAID STRUCTURE BEING UNANNEALED AND HAVING AN ANNEALINGTEMPERATURE HIGHER THAN THE